The present invention relates to a power transmission device for a vehicle, and, more particularly, relates to a power transmission device for a vehicle with two axially opposed driven wheels, in which a differential is provided between these driven wheels and is axially located closer to one of these wheels and further from the other of them--i.e. in an asymmetrical location relative to the wheels.
In the case of a vehicle provided with a transversely mounted engine, especially in the case of a vehicle of the front engine front wheel drive type or the so called FF type, it is very common for the differential device which transmits power from the gearbox of the vehicle to the driven wheels thereof, and which provides differential action between said driven wheels, to be displaced to one side or to the other side of the longitudinal axis of the vehicle, i.e. for this differential device to be closer to one of the driven wheels and further from the other of the driven wheels. This is because of the limited space available in the engine compartment of such a vehicle, due to the transverse mounting of the engine therein, and to the relative alignment of the various units in the power train of the vehicle, and is a well known matter in the automotive art.
The question then arises as to how the rotary power is to be transmitted from such an asymmetrically positioned differential to these driven wheels of the vehicle. Such a differential is usually provided with left and right power output shafts which extend in opposite directions to the left and the right sides thereof. Further, it is an almost inevitable constructional fact that the differential will have a so called preferred power output line, on each of its sides, these lines being typically defined by the rotational axes of some internal members of the differential. (It is common for the preferred power output lines on the left and the right sides of the differential, in fact, to be coincident.) In other words, it will be preferred, for proper operation of the differential, and for good reliability thereof and quietness in operation, that the power output shaft on each side should be maintained with its central rotational axis substantially coincident with the aforesaid preferred power output line on that side. If one or the other of the power output shafts should be undesirably slanted out of the preferred power output line on its side, so that its central rotational axis lies at a substantial non zero angle to said preferred power output line, then the reliability of the differential will be much deteriorated during use, due to excessive side strains being imposed on bearing devices in the differential, and mutual misalignment of gears therein, and further there is a danger that a loud noise such as a humming noise will be caused during operation of the vehicle in which the differential is fitted, which is troublesome and annoying. In a severe case, also, it is possible for quite high amplitude vibration to be generated during operation of the transmission, which of course is completely unacceptable.
If the left and the right power output shafts of the differential are of the same length, and if the end of each of these power output shafts is connected to its respective driven wheel by a dirve shaft and by constant velocity universal joints which may be Cardan joints, then, since the differential is offet towards one side of the vehicle, therefore the left and the right drive shafts will necessarily be of different lengths, and these drive shafts will necessarily bend downwards at different angles, and will be connected to the driven wheels at different angles. This means that during acceleration of the driven wheels, and also, when the driven wheels are the front wheels of the vehicle and are also used for steering the vehicle, during progress around a curve, then the difference in the torques provided to these driven wheels of the vehicle, when the steering geometry changes as it inevitably will in such circumstances, will cause asymmetrical acceleration and steering characteristics for the vehicle. For example, it may be the case that a tendency may arise for the vehicle, during acceleration, to steer towards the side on which the drive shaft is shorter. This deteriorates straight ahead drivability of the vehicle, causes dangerous handling thereof, and accordingly is quite unacceptable.
In view of the above described problem, a constructional solution has been proposed for a power transmission with such an asymmetrically located differential, in which the left and right drive shafts are of equal lengths, and bend downwards at the same angles, and on the side of the vehicle on which the distance between the differential and the driven wheel is the shorter the inner end of the drive shaft is directly connected to the outer end of the power output shaft of the differential by a constant velocity universal joint which may be a Cardan joint, while on the side of the vehicle on which the distance between the differential and the driven wheel is the greater the inner end of the drive shaft is connected to the outer end of an intermediate shaft, the inner end of which is connected to the outer end of the power output shaft of the differential on that side. In this case, this intermediate shaft may be supported, via bearings, by some fixed member of the vehicle.
In such a construction, the angles down through which the left and the right drive shafts incline are the same, and accordingly the left and the right driven wheels are provided with equal steering torques, accordingly eliminating asymmetrical steering and driving characteristics of the vehicle; but on the other hand, because the power output shaft of the differential on the side on which the differential is further from its driven wheel needs to be connected to the intermediate shaft via a third universal joint such as a Cardan joint, in order to compensate for manufacturing tolerances in the alignment of the rotational axis provided for the intermediate shaft by the means for supporting the intermediate shaft, and the rotational axis of the power output shaft on that side of the differential, which as stated above ought to lie substantially along the preferred power output line of the differential on that side, thereby the number of mechanical components required for the transmission is increased, and according to this increase in complexity the cost of the transmission is increased and the difficulty of manufacture thereof becomes higher. Further, the reliability of the transmission is decreased, and further problems arise such as an increase in weight, and an increase in the slack in the power train caused by the additional splined joints in the power train, which causes a risk of producing a clonking noise from the transmission, especially when starting the vehicle from rest. Accordingly, such a solution, although it has been practiced, is by no means perfect.